a) Field of the Invention
The present invention relates to an intermediate frequency filter (will be referred to as "IF filter" hereinafter), and more particularly, to an IF filter using an amplitude-flat monolithic ceramic filter.
b) Prior Art Statement
In a linear modulation system used in digital radio communications, such as quadri-phase shift keying (QPSK), a digital signal having a wide band of spectrum, if modified as it is for radiation from the antenna, will result in an interfering radio wave to the surroundings. To eliminate the interfering radio wave component, the digital signal must be shaped to limit its spectral band without degradation of the information its carries. The digital signal is subject to pulse-shaping as passed through a low-pass filter of which the characteristic is defined by a roll-off function. It is well known that with the roll-off factor being 0.5 so that one half of the pulse-shaping is done by the transmitter while the other half is done by the receiver, a high quality of signal transmission can be ensured, the frequency can be actively utilized and the interference can be effectively suppressed.
The roll-off function determining the characteristic of the low-pass filter destined for the pulse-shaping is not any one which can be implemented when its degree is finite. An approximate Nyquist-transmission system has been proposed and has been proved to be effectively usable in this technical field, which uses as IF filter for a receiver a Transitional Butterworth Thomson (TBT) filter in which the transfer function is expressed with a rational function of which the degree is finite and which is approximate to the roll-off function. It was reported that with the roll-off factor of 0.5, the inter-sign interference can be made small when the TBT parameter m is set within a range of 0.4 to 1.0 in case the degree of the transfer function of an optimal IF filter is eight (cf. "Takami et al "An Approximate Nyquist-transmission for QPSK Digital Mobile Communications", IECE (Institute of Electronics and Communication Engineering of Japan) Technical Report No. RCS90-6").
The IF filter for a conventional receiver used in digital radio communications uses a delay-flat ceramic filter as described in the IECE Technical Report No. CS84-98 (Isao Shimizu "Narrow Band Low Distortion Receiver with Delay-Flat Ceramic Filter for 800 MHz Band Mobile Radio System", pp. 67-72). The characteristics of a ceramic filter incorporating two resonators are shown in Table below. The table shows the results of comparison among four types of commercially available ceramic filters each having a center frequency of 10.7 MHz and a 3-dB bandwidth of 230 kHz. They are listed herein in the order from the poorest delay flatness (top) to the best one (bottom). The upper two are of amplitude-flat types, while the lower two are of delay-flat types. In the conventional receiver, a plurality of delay-flat ceramic filters is used as connected to each other in a ladder form since their characteristic can be converted to that of TBT filter. As shown, however, the better the delay-flatness of the ceramic filter, the larger that insertion loss becomes. Namely, the IF filter using the conventional delay-flat ceramic filter is disadvantageous in that the dynamic range of the IF stage is narrow.
TABLE ______________________________________ Ceramic Filter Characteristics Delay flatness Delay deviation (.mu.sec) Insertion loss (dB) ______________________________________ x 1.4 3.0 .dwnarw. 0.8 4.5 .dwnarw. 0.4 9.0 o 0.1 10.5 ______________________________________
Further, when manufacturing a ceramic filter having a delay flatness, it is necessary to adjust the oscillation frequency of each ceramic oscillator, which will cause the yield of production to be reduced at the time of mass production. This will also raise the manufacturing costs of delay-flat ceramic filter, and thus those of receiver, which will also be an obstacle to the widespreading of digital mobile communications.
Furthermore, since the conventional IF filter has used therein in a ladder form a plurality of delay-flat ceramic filters each incorporating two resonators, the IF filter itself cannot be designed compact and thus it is not suitably usable in a small receiver.